1. Field of the Invention
The present invention relates to a heat treatment method and a heat treatment apparatus for steel products.
2. Description of the Related Art
Atmosphere control is important in heat treatment of a steel product, and such atmosphere control is performed by controlling CP (carbon potential) in a heat treatment atmosphere. Conventionally, there is disclosed a method of stabilizing CP at a constant value by controlling a supply amount of enriched gas (CmHn gas) based on CP during carburization heat treatment of a steel product (Japanese Patent Publication No. Hei 5-15782). There is also disclosed a method of stabilizing CP by feedback control such as proportional control, PID control or the like (Japanese Patent Application Laid-open No. 2003-013136).
However, in a conventional heat treatment furnace, there is a problem such that when an opening of the furnace is opened for carrying a workpiece in or out, air enters inside the furnace and decreases CP largely. Particularly, there is a problem such that when CP is feedback controlled, a control response (CP) overshoots. Furthermore, there are cases such that a control response becomes unstable to cause hunting, or take a long time to reach a target value.